(The mayor was lobbied by the other side last week, when he went golfing with Vice President Joe Biden.)

But does Romney actually expect him to take sides in 2012, after Bloomberg stayed neutral in 2008 in very similar circumstances?

It's clear why Romney would want the Bloomberg endorsement, certainly. The mayor would offer "centrist" validation from a government-interventionist, green-initiative-loving social liberal, and the businessy imprimatur of person who is, in some sense, also a corporation. What Bloomberg's endorsement would tell voters who value his opinion is that the Mitt Romney they saw during primary season, who advocated positions on guns, gay rights, immigration and even taxes that the mayor abhors, was the fake Mitt Romney. Bloomberg would be telling them that it's safe to like Romney, now that Rick Santorum has been dispatched and he is at liberty to stop saying scary things.

But what would Bloomberg get in the bargain?

Perhaps it's the obvious thing—consideration for a position in a Romney administration. But it's not all that clear from anything that mayor has ever said that he would want that, even if it were on offer.

What the mayor and his aides have made clear, however, is that he wants to play a meaningful role in public life after he leaves office, facilitated by his ability to put his billions of dollars in the service of causes he believes in. Significantly, his opposition to partisanship is supposed to enable him to influence whichever of the two major parties happens to be standing in the way of his legislative goals for America.